For several years, PWM amplifiers have been used for power amplification in audio applications as they provide power amplification with a minimum of power loss. The PWM modulation and amplification means however introduces a relatively large amount of noise, errors and distortion making the use of PWM amplifiers only suitable for low-quality applications as e.g. PC-speakers, etc. Furthermore, the PWM modulation requires a very accurate saw-tooth or ramp signal to be present. Imperfections in that reference signal are directly projected as errors on the modulated audio signal.
An additional conventional PWM amplifier has been improved by establishing negative feedback from the output of the power stage back to the input of the modulator. Thereby, most of the errors and noise introduced by the modulation and power stages are suppressed. However, to obtain a usable utility bandwidth the switch frequency of the power stage has to be relatively high, which increases the noise and error introduced by the modulator and power switches. Also the loop filters have to be designed with care to avoid instability. This seriously limits the possible uses of the loop filters.
A further improved PWM amplifier utilizes controlled instability in the feedback loop to omit the reference signal and thereby causing a self-oscillating PWM amplifier. Thus, the possible errors of the reference signal have been avoided. Such self-oscillating PWM amplifier however still present several requirements to the design of the loop filters as the instability is often desired to arise at a certain frequency. Thus, only simple loop filters with limited open loop gain and thus limited noise suppression and relatively narrow utility bands have been provided with PWM amplifiers. Furthermore, subsequent demodulation means also introduces noise and distortion which is not suppressed.
One of several objects of the present invention is to provide a self-oscillating PWM amplifier with improved suppression of noise, distortion and error.
One of several objects is to provide a self-oscillating PWM amplifier with broadened utility bandwidth.